Azo yellow pigments are a class of colorants that are tinctorially strong and relatively inexpensive. Monoazo yellow pigments exhibit good lightfastness and are useful as coloring agents for paints and some printing inks. In plastics, however, monoazo yellow pigments tend to undesirably bloom and discolor at high temperatures, so their use is disfavored. Diarylide disazo yellow pigments, made from 3,3'-dichlorobenzidine, are much stronger tinctorially and show a much lower tendency to bloom and discolor at high temperatures. C.I. Pigment Yellow 17 is a strong green-shade diarylide yellow and has often been used for plastics. W. Herbst and K. Hunger, Industrial Organic Pigments, VCH, New York, 1993, p. 252, state: "The plastics industry, however, uses P.Y. 17 extensively, . . . P.Y. 17 is also frequently used in polyolefins . . . . Its heat stability in these media was said to be about 220.degree. to 240.degree. C." However, R. Az et al reported in Dyes and Pigments, 15, 1 (1991), that diarylide yellows are degraded to potentially carcinogenic by-products (e.g., 3,3'-dichlorobenzidine) in plastics processed above 200.degree. C., a temperature lower than that used in processing most plastics (many plastics are processed at temperatures from 230.degree. C. to 330.degree. C.). In efforts to overcome these disadvantages, azo pigment manufacturers have actively pursued development of other types of azo yellows.
One alternative type of azo yellows include disazo yellow pigments made by coupling bis(acetoacetamido)benzenes (BAAAB) into aniline derivatives. U.S. Pat. No. 5,616,778 to Goldmann et al relates to coupling 1,4-BAAAB into 2,5-dicarbomethoxyaniline (to give C.I. Pigment Yellow 155) and heating the pigment in an organic solvent at 80.degree.-150.degree. C. for up to 6 hours. U.S. Pat. No. 5,559,216 to Jung et al gives 47 Examples and describes coupling 1,4-bisacetoacetamidobenzene (1,4-BAAAB) into dozens of aniline derivatives, but does not mention 2-ethoxyaniline (o-phenetidine). This patent relates to processes for making all these disazo aniline derivative pigments (with a fairly complete list of possible aniline substituents, including C.sub.1 -C.sub.4 alkoxy) in the presence of nonionic surfactants having a 5.degree.-90.degree. C. cloud point to give pigments suitable for printing inks.
German Offenlegungsschrift 3501199 (1985, Sandoz GmbH) relates to coupling 1,4-BAAAB into mixtures of anilines substituted with one or two carboalkoxy groups to give pigments that disperse easily into polypropylene. Canadian Patent 1135688 (1982, Hoechst AG) relates to the pigments resulting from coupling 1,4-BAAAB into anilines substituted with nitro and carboalkoxy groups. Czech Patents 188727 (1981) and 185798 (1978) relate to the pigments resulting from coupling 1,4-BAAAB into anilines substituted with urea groups. U.S. Pat. Nos. 4,146,558 and 4,103,092 to Jefferies et al relate to coupling 1,4-BAAAB into anilines substituted with quaternized aminoalkyl or aminoalkoxy groups to give water-soluble dyes. Swiss Patent 585247 (1977, Sandoz Ltd.) relates to coupling 1,4-BAAAB into an aminoquinoline derivative and heating with N,N-dimethylformamide at 140.degree. C. to obtain a greenish yellow pigment for coloring polyvinylchloride plastics. U.S. Pat. No. 3,978,038 to Cseh et al lists 327 Examples of pigments resulting from coupling 1,4-BAAAB or various substituted 1,4-BAAABs into anilines substituted with nitro groups. U.K. Patents 1400533 and 1396526 (1975, Ciba-Geigy AG) relate to the pigments resulting from coupling 1,4-BAAAB or various substituted 1,4-BAAABs into anilines substituted with a chlorine and methyl groups or two chlorine groups. German Offenlegungsschrift 2336915 (1973, Farbwerke Hoechst AG) relates to the pigments resulting from coupling 1,4-BAAAB or various substituted 1,4-BAAABs into anilines substituted with nitro groups and other groups.
Other alternative types of azo yellows include the metallized monoazo yellows, analogous to the metallized azo red pigments that exhibit high temperature stability in plastics. C.I.Pigment Yellow 62 is a slightly redder yellow, but much weaker tinctorially than C.I. Pigment Yellow 17. Another metallized monoazo yellow, described in U.S. Pat. No. 5,669,967 to Hays, is stronger tinctorially than C.I. Pigment Yellow 17, but is an even redder yellow (though not as red as metallized medium-shade yellows C.I.Pigment Yellows 183 and 191).
Two alternative yellow pigments used by plastics processors include C.I. Pigment Yellow 109 (isoindolinone) and C.I. Pigment Yellow 138 (quinophthalone). These green-shade yellows are very close in hue to C.I. Pigment Yellow 17, but are much more expensive, much weaker and contain eight chlorine atoms per molecule, a potential environmental disadvantage. In this connection, generally speaking, the possibility of degradation into polychlorinated biphenyls (PCBs) and other dangerous compounds exists with aromatic compounds containing chlorine atoms.
Plastics processors using diarylide yellows are particularly conscious of the increased costs of using the higher concentrations of weaker yellows required for coloring plastics to the required tinctorial strengths. Although C.I. Pigment Yellow 155, a disazo pigment made from coupling 1,4-BAAAB, is available, this pigment (Sandorin 4G) is much weaker and slightly redder than C.I. Pigment Yellow 17 and shows poor heat stability (see below) at 288.degree. C., a temperature commonly used to process polypropylene and other plastics. Thus, there is a need for new green-shade yellow azo pigments that exhibit strength comparable to C.I. Pigment Yellow 17, exhibit good heat stability and good lightfastness, and present little or no adverse environmental impact.